回答:
textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String)
txt_Password = new UITextField {
Frame = new RectangleF (20,40,180,31),
BorderStyle = UITextBorderStyle.Bezel,
TextColor = UIColor.Black,
SecureTextEntry = true,
Font = UIFont.SystemFontOfSize (17f),
Placeholder = "Enter Password",
BackgroundColor = UIColor.White,
AutocorrectionType = UITextAutocorrectionType.No,
KeyboardType = UIKeyboardType.Default,
ReturnKeyType = UIReturnKeyType.Done,
ClearButtonMode = UITextFieldViewMode.WhileEditing,
};
secureTextEntryをtrueに設定します。